We have cloned a protein, RBAP2, based upon its ability to bind to the retinoblastoma gene product in vitro. We have extended the clone to full- length status, shown that it is a nuclear phospho- protein, and that it binds to RB in vivo. This binding follows the same genetics as has been established for the interaction of RB and the viral oncoproteins large T antigen, E1A, and E7. This grant now proposes to investigate the biochemical and biological functions of RBAP2 and, in addition, the role of RBAP2 in RB mediated cell-cycle regulation.